Document images can be scanned, and electronic images thereof can be generated and stored, at resolutions such as 2400 dots per inch that are far beyond what can be displayed on monochrome cathode ray tube ("CRT") devices that are available today at reasonable cost. Presently, scanned document image displays provide screen images with resolutions of the order of one hundred dots per inch or less.
Cinque et al., in U.S. Pat. No. 4,210,936, disclose method and apparatus to reproduce a half-tone image by dividing the image into a number of pixels in which one image data are compressed; the image reproduction matrix utilizes less gray scale data than is generated from the original pixel matrix. In one embodiment, four information bits of gray scale data are combined to yield one information bit in the displayed image. Apparently, no technique is disclosed for enhancing the actual or perceived resolution of the compressed gray scale data through modification of the image display.
U.S. Pat. No. 4,229,768, issued to Kurahayashi et al. is one of the earliest patents to disclose use of a protocol that later became the CCITT X.25 standard. A high data compression ratio is achieved without excessive deterioration in image quality by use of run-length encoding for data compression and an inverse process for data expansion, using correlation of image signals on adjacent scan lines of the original image.
A coarse scan and fine print algorithm is disclosed by Bacon in U.S. Pat. No. 4,280,144. The original coarsely scanned pixel element is divided into four adjacent sub-pixels of approximately equal area, and pixel information for the original pixel and for four nearest neighbor pixels is used to construct modified pixel values for each of the sub-pixel regions contained in the original pixel. Each reconstructed pixel value thus relies upon data from three adjacent scan lines.
Pennebaker discloses combination of pixel value data of two adjacent pixels on a scan line in order to construct composite pixels with improved image quality in U.S. Pat. No. 4,532,503. Each composite pixel value is defined by two bits, with the most significant bit being defined only by the original pixel value and the least significant bit of the composite pixel value being determined by the two neighboring pixel values. The composite pixel value contains information from only a single scan line.
In U.S. Pat. No. 4,532,651, Pennebaker et al. disclose a gray scale filter for image data by combining pixel values from two adjacent pixels on a single scan line of the original image. The most significant bit of a pixel value is used to determine gray scale transition in a reconstructed image.
An object of the invention is to provide scanned document images at a perceived resolution of up to 2400 dots per inch using a raster scan CRT display.
Another object of the invention is to provide means for electronic generation of document images that provide a reasonable gray scale for static image display without requiring excessive band width.